The printed wiring board remains today as to the most common device for interconnecting electronic components to each other. Most present-day printed wiring boards are comprised of one or more layers of a glass epoxy or polyester material such as FR-4 or the like. On the exposed surfaces of one or both of the two outer layers of the printed wiring board is a pattern of metallization comprised of metallized component-receiving areas and metallized links. Some of the links may be connected to the component-receiving areas, which are adapted to receive a conductive member of a component for bonding thereto. Each intermediate layer also has a pattern of metallized links which run between metallized vias. Such vias electrically connect a metallized link on one layer to a selected link on an adjacent layer or to a component-receiving area on one of the outer layers. In this way, the links on the outer and intermediate layers to connect a pair of component-receiving layers so as to create a "wiring path" therebetween.
There is currently a trend towards increasing the density of circuits within active electronic components (i.e., integrated circuit devices) which has led to an increase in input/output connections that must be made to such components. The increase in input/output connections generally requires an increase in the number of wiring paths that must be provided between such components by the printed wiring board. The conventional approach to increasing the number of wiring paths provided by a given-size printed wiring board is to increase the number of layers and to reduce the linewidth of the metallized links on each layer. As a practical matter, the linewidth of the links cannot be reduced below a minimum level because of constraints associated with proper signal transmission, current carrying capability, and fabrication limitations. Once the linewidth of the links has been reduced to this minimum level, then any increase in the number of wiring paths must be accomplished by increasing the number of printed wiring board layers.
Unfortunately, increasing the number of layers of a printed wiring board greatly increases the board fabrication cost. Therefore, it is undesirable to fabricate a printed wiring board having more layers than are necessary to provide the requisite number of wiring paths. In many instances, the number of wiring paths required in certain regions of the printed wiring board is less than in other regions. However, there is currently no practical way to build printed wiring boards having varying numbers of layers in different regions. Therefore, each printed wiring board must be fabricated with a sufficient number of layers to provide the maximum number of wiring paths required anywhere on the board, thus resulting in high fabrication costs.
Thus, there is a need for a printed wiring board which provides for an increased number of wiring paths in selected regions of the board.